Tectonic Evolution of the Baiyinduxi Group: Insights from Detrital Zircon Geochronology, Bainaimiao, Inner Mongolia, China

Baiyinduxi group is located between the Bainaimiao magmatic arc to the South and Wenduermiao complex to the North geographically. It is to the South of the proposed Suolun suture zone between Siberian plate and the North China plate, and to the North of the Early-Middle Paleozoic Bainaimiao accretionary wedge. Main rocks in this complex are moderate grade metamorphic rocks with felsic protolith (Xiao et al. 2003; Windley et al. 2007; Jian et al. 2008). Study of this complex will shed light on the tectonic evolution of the northern margin of the North China plate. Previous workers proposed that this group is metamorphosed Pre-Cambrian crystalline basement or geosyncline deposits based on limited Sm-Nd isotope data. Some other workers proposed that it was formed as foreland basin deposits. In order to determine the accurate age of Baiyinduxi group, we collected two quartzite samples from this group. 100 zircons from each sample have been analysed using LA-ICP-MS method. All these zircon grains are relatively small and lack of erosion, which indicates that they are close to the source area. Almost all zircons show zoning features that usually happened to igneous origin grains, while there are a few that have a very thin outermost metamorphic layer. This tells us that most zircons are the weathering products of igneous rocks, which are not far away from the Baiyinduxi group. The analyses yield ages between 335 Ma -518 Ma. All ages are well aligned along the U/Pb concordia and clustered. These ages can represent the crystallization age of the protolith of this complex and the oldest age of the deposition. The peaks of the ages are at 480 Ma and 450 Ma, while there are two gaps at 430 Ma and 370 Ma. This age distribution pattern agrees well with the ages of plutonic and volcanic rocks in the magmatic arc nearby (unpublished data from Liu et al), which indicates that the arc rocks are the source of the this sedimentary deposit and the protolith of the this group can be the fore-arc sedimentary formation which formed at the same time when the Pleo-Asian Sea floor subducted beneath the North China Plate. There are no syn-collision zircons found in these two samples, so the protolith of this group should form before the final collision (280 Ma) happened and between 335 Ma -280 Ma.